1. Field of the Invention
The present invention relates to a cleaning apparatus and a substrate cleaning method to clean a substrate consisting of a transparent material such as a glass substrate. More particularly, it relates to an apparatus for cleaning a substrate and a method for cleaning a substrate, capable of efficiently removing an organic type or metallic type contaminant adhered to the substrate surface.
2. Discussion of Background
Irradiation of a substrate surface with ultraviolet light or vacuum ultraviolet light (hereinafter referred to as the UV light) is one of effective cleaning techniques to remove an organic type contaminant or residue from the substrate surface thereby to modify the substrate surface from hydrophobic to hydrophilic. Here, the contaminant is meant for a convex fouling present on the substrate surface, such as particles or fibers, and the residue is meant for an organic type material adsorbed on the substrate surface. Further, as the UV light source for emitting the UV light at the time of cleaning the substrate, a dielectric barrier discharge excimer lamp filled with xenon gas and a mercury lamp may be used. Hereinafter, the UV light in this specification is meant for UV light having a wavelength of from 100 to 400 nm.
A xenon (Xe) excimer lamp, more specifically a dielectric barrier discharge excimer lamp filled with xenon gas, emits a UV light having a center wavelength of 172 nm. The light of 172 nm will be intensely absorbed by oxygen molecules O2 in the atmosphere, whereupon excited state oxygen atoms O(1D) will be formed directly (the following formula (1)) or via ozone O3 (the following formulae (2) and (3)).
Xe2 excimer lamp:O2+hν(172 nm)→O(1D)+O(3P)  Formula (1)O(3P)+O2→O3  Formula (2)O3+hν(172 nm)→O(1D)+O2  Formula (3)
On the other hand, a low pressure mercury lamp emits UV lights having center wavelengths at 185 nm and 254 nm. The UV light with a wavelength of 185 nm will be absorbed by oxygen molecules O2 in the atmosphere, whereupon ozone O3 will be generated (the following formulae (4) and (5)). This ozone O3 will absorb the UV light of 254 nm to form excited state oxygen atoms O(1D) (the following formula (6)).
Low pressure mercury lamp:O2+hν(185 nm)→O(3P)+O(3P)  Formula (4)O(3P)+O2→O3  Formula (5)O3+hν(254 nm)→O(1D)+O2  Formula (6)
Excited state oxygen atoms O(1D) and ozone O3 will readily oxidize and decompose an organic compound into CO, CO2 and H2O, whereby the organic type contaminant and residue present on the substrate surface will be removed.
Further, when an aqueous solution or gas containing ozone is irradiated with a UV light within a wavelength range from 240 to 260 nm wherein ozone has an absorption band, active oxygen species having an oxidizing force higher than ozone (such as superoxide O2−, hydroperoxide HO2, hydroxyl radical OH., excited oxygen O2*, etc.) will be formed. For example, the oxidation-reduction potential of hydroxyl radicals OH. is 2.85 eV, which is higher than that of ozone, 2.07 eV, and thus, an organic type contaminant or residue can be removed more effectively by using the active oxygen species.
Further, when an aqueous solution or a gas containing oxygen gas is irradiated with light having a wavelength of at most 200 nm, excited state oxygen atoms and ozone will be formed, and then or at the same time, if it is irradiated with a UV light within a wavelength range from 240 to 260 nm, or at most 180 nm wherein ozone has an absorption band, the above-mentioned active oxygen species will be formed, whereby it is possible to remove an organic type contaminant or residue effectively.
Further, when an aqueous solution having hydrogen peroxide dissolved therein, is irradiated with UV light having a wavelength of at most 280 nm wherein hydrogen peroxide shows a light absorption, active oxygen species having an oxidizing force higher than hydrogen peroxide such as super oxide O2−, hydroperoxide HO2, hydroxyl radicals OH., excited oxygen O2*, etc. will be formed. The oxidation-reduction potential of hydrogen peroxide is 1.78 eV, and hydroxyl radicals being one type of active oxygen species have a higher oxidation-reduction potential. Accordingly, the active oxygen species are capable of removing an organic type contaminant or residue more effectively than hydrogen peroxide.
At the same time, the UV light has a photon energy high enough to cut a molecular bond of an organic compound. Such a dissociation process of a molecular bond by the UV light will promote the decomposition of an organic compound to such gases as CO, CO2 and H2O.
In view of the above-mentioned mechanism, in order to carry out cleaning by means of the UV light effectively, presence of ozone O3, excited state oxygen atoms O(1D) or active oxygen species is required, and to generate a substance which promotes such cleaning, it is essential to irradiate an atmosphere wherein at least one of oxygen molecules O2, water molecules, ozone O3 and hydrogen peroxide, is present, with a UV light having a wavelength to be thereby absorbed.
FIG. 4 is a schematic view showing a construction of a conventional substrate cleaning method and substrate cleaning apparatus employing UV light. A substrate cleaning method and substrate cleaning apparatus employing UV light of the construction of this type have heretofore been proposed (Patent Documents 1 to 4).
In FIG. 4, the substrate cleaning apparatus 100 has a process chamber 110 accommodating a substrate 200 to be cleaned, and an UV light source chamber 120 accommodating an UV light source 122 which emits UV light. The UV light source 122 is an UV light source which emits UV light, such as a xenon (Xe) excimer lamp or a low pressure mercury lamp.
In the process chamber 110, the substrate 200 is positioned at a predetermined distance (usually from 1 to 10 mm) from the UV light source 122 and supported by a substrate holder 112 so that its surface 200a to be cleaned will face the UV light source 122. The process chamber 110 has an inlet 114 and outlet 116 for a process gas. The inlet 114 and outlet 116 are used to let a process gas flow in the process chamber 110 when the substrate 200 is to be cleaned. As the process gas, a gas containing water vapor or O2 is usually employed. Specifically, O2 gas, a mixed gas of O2 and N2, humidified N2 gas or air is, for example, employed.
On the other hand, the UV light source chamber 120 has an inlet 124 and outlet 126 for an inert gas. The inlet 124 and outlet 126 are used to let an inert gas having a low absorbance at the wavelength of UV light to be used, such as N2, Ar or He, flow in the UV light source chamber 120. By letting the inert gas flow in the UV light source chamber 120, attenuation of the UV light emitted from a lamp 122 before it reaches the process chamber 110, will be prevented.
A window 128 is provided between the process chamber 110 and the UV light source chamber 120. The window 128 is made of a material having a low absorption at the wavelength of UV light to be used, such as quartz glass. The UV light emitted from the lamp 122 will pass through the window 128 and enter into the process chamber 110.
Accordingly, in the case of such a conventional substrate cleaning apparatus 100, the process chamber 110, more specifically, the space between the window 128 and the surface 200a of the substrate 200 to be cleaned, is filled with a process gas containing water vapor or O2. Water vapor or O2 gas has a strong absorption in an ultraviolet light region and a vacuum ultraviolet light region (for example, the absorption cross-sectional area of O2 molecules at 172 nm is 6×1017 cm2), and the UV light accordingly attenuates sharply as it approaches the surface 200a of the substrate 200 to be cleaned, whereby the intensity of the UV light will decrease. In a case where the process gas flowing in the process chamber 110 is a laminar flow, due to the decrease of the UV light intensity, the concentration of O3, excited state oxygen atoms and active oxygen also decrease as it approaches the surface 200a of the substrate 200 to be cleaned.
FIG. 5 is a graph showing a distribution of the intensity of UV light, and the concentrations of ozone O3, excited state oxygen atoms O(1D) and active oxygen in a conventional substrate cleaning apparatus, in the region between the window 128 and the surface 200a of the substrate 200 to be cleaned. As is evident from FIG. 5, the intensity of UV light and the concentrations of ozone O3 and excited state oxygen atoms O(1D) are the maximum in the vicinity of the window 128 and rapidly decrease as they approach the surface 200a of the substrate 200 to be cleaned.
In order to obtain a high intensity of UV light at the surface 200a to be cleaned, it is necessary to lower the concentration of water vapor or O2 gas in the process gas, but if such a concentration is lowered, the concentrations of ozone O3, excited state oxygen atoms and active oxygen at the surface 200a to be cleaned will also decrease.
Further, in order to have the surface 200a of the substrate 200 to be cleaned, irradiated with UV light with a sufficiently high irradiation intensity, it is necessary to emit UV light of high intensity to the window 128. Accordingly, window 128 begins to have absorption due to high intensity UV light and needs to be changed periodically to maintain a sufficiently high irradiation intensity of UV light at the surface 200a to be cleaned.
Also in a case where the flow of the process gas is a turbulent flow, ozone O3, excited state oxygen atoms O(1D) and active oxygen will have short lifetime and can not reach the surface 200a of the substrate 200 to be cleaned.
Consequently, at the surface 200a of the substrate 200 to be cleaned, the intensity of UV light and the concentrations of ozone O3, excited state oxygen atoms O(1D) and active oxygen, are not enough to effectively clean the surface 200a to be cleaned.
Further, according to cited document 1, in order to remove an organic substance effectively from the surface to be cleaned, it is necessary to supply both UV light and ozone gas, excited state oxygen atoms or active oxygen simultaneously to the surface to be cleaned. With only one of them, in other words, irradiation with UV light in an inert gas atmosphere not containing ozone gas, excited state oxygen atoms or active oxygen, or exposure to an atmosphere in which only ozone gas, excited oxygen atoms or active oxygen is present without UV light irradiation, it is not possible to sufficiently remove an organic substance from the surface to be cleaned.
Patent Document 1: JP-A-2002-16033
Patent Document 2: JP-A-2001-137800
Patent Document 3: U.S. Pat. No. 6,507,031
Patent Document 4: JP-A-2002-192089
Cited Document 1: Journal of Illuminating
Engineering Institute of Japan, vol 83, No. 5, p. 273